Microwaves occupy a region in the electromagnetic spectrum between radio waves on the side of lower frequency and infrared waves on the side of higher frequency. While there may be no definite boundary between these regions, electromagnetic radiation having frequencies in the range of 10.sup.9 Hz to 10.sup.11 Hz are normally termed microwaves. Such radiation has wavelengths in the range of about one millimeter to one decimeter. Two specific frequencies--915 MHz and 2450 MHz--have been designated for domestic and industrial heating application.
It is known that such radiation of sufficient intensity will heat compositions that have a suitable relative permittivity, .epsilon..sub.r, and dissipation factor, D. The heating effect is induced within the material. The heat does not have to be conducted to and into the material as occurs by radiant or convective heating in a gas-fired or electric oven, furnace or kiln. Consequently, the total energy consumption is reduced because there is less time during which reradiation losses can occur. For this reason microwave heating, where applicable, offers an energy efficient means of carrying out operations such as drying, baking, resin curing, calcining, sintering and the like. The development of economical, efficient and reliable continuous wave microwave generators, such as the klystron and magnetron, have made microwave heating practical.
A potential application for microwave heating is in the sintering of alumina spark plug insulator bodies. In the green (unsintered) state these bodies are a compacted blend of several inorganic powders--(mostly alumina, Al.sub.2 O.sub.3) and several organic binders--mostly wax. Millions of these hollow, round, elongated bodies of familiar shape are sintered each work day in large gas-fired kilns. Batches of thousands of the bodies are slowly heated from room temperature to about 2900.degree. F. (1593.degree. C.), soaked at that temperature and then slowly cooled. During this long cycle (about 24 hours) the bodies are dried, the wax burned out and the residual ceramic powder sintered into strong unitary bodies. This venerable practice consumes much natural gas, and requires large furnaces and much floor space.
Microwave heating can improve the efficiency of sintering of such ceramic workpieces in all of the above aspects. However, I have found that close control must be had over the rate of introduction of microwave energy into the work load to best realize these advantages. The application of too much microwave power can, e.g., accelerate inherent exothermic chemical reactions in the work load material, causing the temperature to shoot up over the ultimate desired level and possibly inflict damage to equipment and produce unacceptable products. If the energy is applied at too low a rate to avoid such problems, the potential benefit of the microwave heating is not fully realized.